1T-Phase Tungsten Chalcogenides - ACS Publications - American

Article Cite This: ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX

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1T-Phase Tungsten Chalcogenides (WS2, WSe2, WTe2) Decorated with TiO2 Nanoplatelets with Enhanced Electron Transfer Activity for Biosensing Applications Elham Rahmanian,† Carmen C. Mayorga-Martinez,† Rasoul Malekfar,‡ Jan Luxa,† Zdenek Sofer,† and Martin Pumera*,† †

Center for Advanced Functional Nanorobots, Department of Inorganic Chemistry, University of Chemistry and Technology Prague, Technická 5, 166 28 Prague 6, Czech Republic ‡ Physics Department, Faculty of Basic Sciences, Tarbiat Modares University, P.O. Box 14115-175, Tehran, I. R. Iran ACS Appl. Nano Mater. Downloaded from pubs.acs.org by YORK UNIV on 12/11/18. For personal use only.

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ABSTRACT: Layered transition metal dichalcogenides (TMDs) have received a great deal of attention due to fact that they have varied band gap, depending on their metal/chalcogen composition and on the crystal structure. Furthermore, these materials demonstrate great potential application in a myriad of electrochemical technologies. Heterogeneous electron transfer (HET) abilities of TMD materials toward redox-active molecules occupy a key role in their suitability for electrochemical devices. Herein, we introduce a promising biosensing strategy based on improved heterogeneous electron transfer rate of WS2, WSe2, and WTe2 nanosheets exfoliated using tert-butyllithium (t-BuLi) and n-butyllithium (n-BuLi) intercalators decorated with vertically aligned TiO2 nanoplatelets. By comparison of all the nanohybrids, decoration of TiO2 on t-BuLi WS2 (TiO2@t-BuLi WS2) results in the fastest HET rate of 5.39 × 10−3 cm s−1 toward ferri/ferrocyanide redox couple. In addition, the implications of decorating tungsten dichalcogenides (WX2) with TiO2 nanoplatelets in enzymatic biosensor applications for H2O2 detection are explored. TiO2@t-BuLi WS2 outperforms all other nanohybrid counterparts and is demonstrated to be an outstanding sensing platform in enzyme-based biosensor with wide linear range, low detection limit, and high selectivity. Such conceptually new electrocatalytic detection systems shall find the way to the next generation biosensors. KEYWORDS: transition metal dichalcogenides, nanosheets, heterogeneous electron transfer, TiO2 nanoplatelets, H2O2 detection

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INTRODUCTION The emergence of ultrathin two-dimensional (2D) layered materials with unique and promising properties arising from the reduced dimensionality and quantum confinement effects has opened up new frontiers in nanoscience.1−5 Amidst the broad family of 2D materials, ultrathin layers of transition metal dichalcogenides (TMDs), particularly group VI dichalcogenides, have been the center of attention and pursued relentlessly due to their prominent optical, electronic, chemical, and catalytic features which vastly differ from their bulk counterparts.6−9 Owing to these outstanding features, 2D TMDs have been employed in a variety of innovative technologies. Of particular note is their implementation in electrochemical devices for energy harvesting, energy storage, and sensing.10−13 As a consequence of growing interest to explore 2D TMDs for a myriad of electrochemical technologies, gaining an indepth insight into the electrochemistry of these materials as well as developing new strategies to increase their functionalities seems to be indispensable.14,15 The key feature that determines the suitability of 2D TMDs in the field of © XXXX American Chemical Society

electrochemistry is their heterogeneous electron transfer (HET) activity toward redox-active species in a solution.16 A fast HET rate is highly desirable as it effectively accelerates the redox processes for chemical species that require large overpotentials to be oxidized/reduced and hence significantly lowers the energy costs in the industrial production of electrochemical devices.17−19 Improving electron transfer kinetics of 2D TMDs has been pursued by different strategies such as phase transition from semiconductive 2H to metallic 1T phase or by electrochemical pretreatment. 16,20 A recent study by our group has demonstrated electrochemical activation of the HET abilities of MoS2, MoSe2, and WSe2 via reductive treatments at identified reductive potentials based on their respective inherent electrochemistry.21 Another promising approach toward this goal can be decoration of 2D TMDs, in particular their metallic 1T phases, with other nanostructures. This Received: October 11, 2018 Accepted: November 26, 2018

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DOI: 10.1021/acsanm.8b01796 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX

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ACS Applied Nano Materials

Scheme 1. (A) Schematic Representation of the Synthesis Procedure To Obtain TiO2@WX2 Nanohybrid Using Oppositely Charged WX2 and TiO2 Nanosheets as Building Blocks and (B) Schematic Illustration of the Fabrication Process of H2O2 Biosensor Based on TiO2@WX2 Nanohybrids

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RESULTS AND DISCUSSION In line with our first objective of constructing the hybrid materials, WX2 nanosheets were successfully obtained using lithium intercalants of varying strengths, namely, n-butyllithium (n-BuLi) and tert-butyllithium (t-BuLi). Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were utilized to examine the surface morphologies of bulk WX2 materials and their exfoliated counterparts. From the SEM images (Figure S1), large crystallites with lateral dimensions of up to tens of micrometers are observed for untreated WX2 bulk materials. For the n-BuLitreated samples, a significant decrease in average lateral dimension of all WX2 materials was achieved. Notably, in contrast to regularly shaped crystallites of the starting materials, wrinkled nanosheets are observed, which can be assigned to the harsh delamination conditions. The best exfoliation efficiency was observed using n-BuLi in the case of WS2. Despite the presence of large number of small sheets, some relatively large stacked polygons are also present in the samples. In the case of t-BuLi-treated WX2, more exfoliated flakes with wrinkled morphologies were obtained particularly in the case of WS2. These results clearly indicate that t-BuLi is more effective organolithium compound in exfoliating WX2 into small nanosheets compared to n-BuLi. Further structural characterization of exfoliated WX2 nanosheets was performed with TEM and high-resolution TEM (HRTEM). As can be observed from Figure 1, the presence of individual few-layer sheets confirms the successful exfoliation

methodology has been widely employed in the case of graphene. For instance, Mao and co-workers has recently demonstrated a remarkable enhancement in HET activity of reduced graphene oxide by utilizing carbon nanotube as a highly conductive support material.22 Among various nanostructured materials, TiO2 nanoplatelets have been demonstrated to be a promising candidate for a variety of electrochemical purposes owing to the electrochemically active Ti ions, inherently high surface-to-volume ratio, biocompatibility, and long-term stability.23 Although TiO2-2D TMD nanohybrid materials have been widely explored for photocatalytic applications,24−30 the HET performance of these nanohybrid materials has not been investigated yet. Therefore, herein, we report on charge transfer ability of 2D TMDs decorated with TiO2 nanoplatelets (Scheme 1A). Toward this aim, we investigate the effects of decorating the family of tungsten chalcogenides with a general formula of WX2, where X = S, Se, and Te, with TiO2 nanoplatelets to enhance their electron transfer kinetics. In addition, optimizing the HET activity of hybrid materials is pursued by changing the weight ratios of constructing components and using of WX2 which have been obtained via different chemical exfoliation methods. Finally, the electrochemical viability of these hybrid materials toward the enzymatic detection of H2O2 is examined (Scheme 1B). B


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Figure 2. Raman spectra of bulk, n-BuLi exfoliated, and t-BuLi exfoliated WS2, WSe2, and WTe2 materials.

for bulk to 13 and 14 cm−1 for n-BuLi-treated and t-BuLitreated samples, respectively. Nevertheless, no obvious peak broadening was observed for the A1g vibrational mode. Furthermore, as another measure of successful exfoliation, the relative intensity of E12g peak to A1g peak, i.e., IE12g/IA1g, increases for treated materials in comparison to bulk WS2, consistent with earlier studies.32,33 In the case of WSe2, it has been reported that the frequency difference between the E12g and A1g Raman modes depends on the number of layers, which is about 3 cm−1 in the bulk state and reaches to 11 cm−1 in the monolayer limit. As can be seen, for the starting material, these two modes appear as one peak with a shoulder. In contrast for the treated samples, shoulder peak becomes more pronounced which indicates the separation of vibrational modes and thus proves the efficient delamination of the WSe2 into a 2D configuration.34 Finally, in the case of WTe2, four characteristic Raman modes of A91, A71, A41, and A31 are observed with corresponding peak positions of 213, 164, 143, and 124 cm−1, respectively. While a significant reduction in the intensity of A91 vibrational mode occurs in both organolithium-treated sample, the other three modes of A71, A41, and A31 exhibit an increase in intensity. In addition, broadening and softening is observed which has been assigned to phonon confinement and the decrease in lateral dimensions of the nanosheets.35 X-ray diffraction (XRD) analysis was employed to determine the crystallographic structure and the phase purity of bulk and delaminated WX2 materials. As can be seen from Figure S2, a strong (00l) diffraction peak is present in the diffraction pattern of all WX2 samples, indicating their layered structure. In addition, no impurity related reflection is observed, implying the high purity of exfoliated materials. As another constructing component of the hybrid material, ultrathin 2D nanoplatelets of TiO2 were synthesized by previously reported surfactant self-assembly approach.36 From the SEM micrograph (Figure 3A), the obtained TiO2 exists in the form of groups of nanoplatelets with rolled-up edges as a result of surface tension. The size of these nanoplatelets is in the range of 100 nm which is much smaller than the size of lithium intercalated WX2. This difference is a natural consequence of utilizing different approaches in the synthesis of TiO2 and WX2 and provides the opportunity to decorate the surface of WX2 with large number of TiO2 nanoplatelets to tune their intrinsic properties. However, generally, a majority of 2D materials carry negative charge on their surfaces originating from their intrinsic stoichiometry or chemical modification during their synthesis process.37 In

Figure 1. TEM and HRTEM micrographs of t-BuLi/n-BuLi exfoliated WX2 nanosheets with the corresponding SAED patterns.

process. Consistent with SEM results, the smaller lateral size of t-BuLi-treated nanosheets compared to n-BuLi-treated ones is observed proving the better performance of t-BuLi in delamination of WX2 materials. Furthermore, the atomic arrangements shown in HRTEM images illustrate the presence of both honeycomb and hexagonal lattices corresponding to 1T and 2H polymorphs, respectively. However, the structural conversion of 2H to 1T phase in t-BuLi-treated samples is more evident compared to n-BuLi-treated WX2 materials. Moreover, Raman spectroscopy was also performed to further explore the level of exfoliation of WX2 nanosheets. Figure 2 summarizes and compares the Raman spectra of bulk WX2 materials and their exfoliated counterparts. First for WS2, all samples exhibited two characteristic peaks centered at about 352 and 421 cm−1 which correspond to the in-plane E12g and vertical plane A1g vibrational modes, respectively, which is in good agreement with previous reports.30,31 Moreover, the E12g mode of WS2 broadens after exfoliation, with an obvious increase in full width at half-maximum (fwhm) from 11 cm−1 C


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For this aim, 3 mm glassy carbon (GC) electrode was modified with the desired TiO2@WX2 nanocomposite. HET rate constants were then derived from peak-to-peak separation of the voltammetric responses by utilizing Nicholson’s approach where larger separation between the anodic and cathodic peaks relates to slower HET rates.40 Parts A−C of Figure 5 represent the voltammetric responses of TiO2@WX2 nanohybrids in the presence of 5 mM [Fe(CN)6]3−/4− with 0.1 M KCl. Figure 5A reflects well-defined redox peaks with a peak-topeak separation (ΔEp) of 161 mV, 144 mV, 116 mV, 85 mV, and 81 mV for n-BuLi WS2, t-BuLi WS2, TiO2, TiO2@n-BuLi WS2, and TiO2@t-BuLi WS2, respectively. These results clearly indicate that t-BuLi WS2 nanosheets decorated with TiO2 exhibit notably lower ΔEp values and hence faster HET rates in comparison to the constructing counterparts. Interestingly, the same trend in HET rates and current responses was observed for TiO2 decorated WSe2 and WTe2 nanosheets (Figure 5B, Figure 5C). The high activity of t-BuLi WX2 can be attributed to their predominant 1T-phase polymorph crystal structure (see Figure 1) that enhances their electrical conductivity as well as increased density of catalytic active sites, on both basal plane and edges. These 1T-phase WX2 materials represent suitable supports for TiO2 nanoplatelets; the resulting nanocomposites have shown improved HET capability mainly due to their enhanced surface area. On the basis of these results, this remarkably improved electrochemical response of TiO2@WX2 nanohybrids could be attributed to the combination of several factors. First, WX2 nanosheets with planar morphology and large surface area could act as ideal platforms for homogeneous decoration of vertically aligned TiO2 nanoplatelets, thus preventing their aggregation. Second, the strong electrostatic interaction between WX2 nanosheets and decorated TiO2 nanoplatelets could provide robust and intimate contact between them, thereby effectively facilitating electron transfer process. Third, vertical orientation of TiO2 nanoplatelets could lead to an enlarged surface area with increased number of electrochemically active sites which provides direct pathways for electron movement to the electrode. Finally, metallic nature of 2D WX2 arising from the synthetic method is highly desirable as it can accelerate the electron transfer process. Upon closer scrutiny of Figure 5D, we discover two distinct trends in the HET rates and current responses of TiO2@WX2 nanohybrids. The first is a dependence on exfoliation method of WX2 nanosheets, where TiO2@WX2 nanohybrids with tBuLi-exfoliated WX2 nanosheets exhibit faster HET rates and increased peak currents in comparison to their n-BuLi-treated counterparts. For instance, TiO2@t-BuLi WS2 has the k0obs value of 5.39 × 10−3 cm s−1, slightly better than that of TiO2@ n-BuLi WS2 at 5.12 × 10−3 cm s−1 (see Table S1). In a similar fashion, TiO2@t-BuLi WSe2 and TiO2@t-BuLi WTe2 afforded faster HET rates (5.17 × 10−3 and 5.05 × 10−3 cm s−1, respectively) when compared to the corresponding n-BuLitreated counterparts, i.e., TiO2@n-BuLi WSe2 (k0obs = 4.95 × 10−3 cm s−1) and TiO2@n-BuLi WTe2 (k0obs = 4.84 × 10−3 cm s−1). The enhancement in HET could be ascribed to the enhanced surface to volume ratio as a consequence of the delamination method, which is in good consentient with previous studies.15 The second observed trend is dependence of electrochemical activity on the type of chalcogen atom, where WS2-based nanohybrids demonstrate faster HET rates and current intensities, followed by WSe2 and WTe2.

Figure 3. (A) SEM images of ultrathin 2D nanoplatelets of TiO2 at different magnifications. The scale bars correspond to 100 nm. (B) ζ potential measurements for TiO2 nanoplatelets before and after modification with PEI, n-BuLi-exfoliated WX2 nanosheets with corresponding TiO2 decorated nanohybrids, and t-BuLi-exfoliated WX2 nanosheets with corresponding TiO2 decorated nanohybrids.

order to verify the aforementioned hypothesis, ζ potential measurements were carried out for all the constructing components (Figure 3B). As expected, the ζ potential values of all WX2 and TiO2 suspensions are negative which confirm the presence of negative charges on their surfaces. This issue hampers precisely controlled decoration of TiO2 on the surface of 2D WX2 via the electrostatic forces between them. A fascinating approach to tackle this obstacle is to modify the surface charge of one of the anionic building blocks to be positive using the high molecular weight polycations.38 Toward this goal, TiO2 was selected as a candidate for surface charge modification using high molecular weight polyethylenimine (PEI). After blending TiO2 with PEI solution, no obvious sedimentations were observed in the obtained sample, demonstrating that the adsorbed PEI polycations have strongly attached onto the surface of TiO2 nanoplatelets. The cationic nature of the PEI-modified TiO2 suspension was investigated using the ζ potential measurement. As can be seen in Figure 3B, the ζ potential value of PEI-modified TiO2 is positive, contrary to the negative charge of the unmodified TiO2 nanoplatelets. This charge modification process enables decoration of positively charged TiO2 nanoplatelets on the surface of negatively charged WX2 through spontaneous electrostatic flocculation procedure (Scheme 1A). The flocculation of TiO2 with WX2 was performed by mixing their aqueous suspensions.39 Figure 4 illustrates typical transmission electron microscopy (TEM) images of TiO2@WX2 flocculated products and their corresponding energy-dispersive X-ray spectroscopy (EDX) elemental mappings. TEM images clearly demonstrate that ultrasmall TiO2 nanoplatelets have anchored on the surface of all WX2. In addition, the EDX results reveal that Ti, W, and respected chalcogen atom elemental distributions are overlapped highly, indicating uniform dispersion of TiO2 nanoplatelets in the TiO2@WX2. Heterogeneous Electron Transfer Properties of TiO2@ WX2 Nanohybrids. Moving to investigate the effect of TiO2 decoration on the HET properties of WX2, cyclic voltammetry was employed using ferri/ferrocyanide [Fe(CN)6]3−/4− redox couple in the presence of supporting electrolyte 0.1 M KCl. D


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Figure 4. TEM images of TiO2@WX2 nanohybrids and corresponding energy-dispersive X-ray spectroscopy (EDX) elemental mapping images of titanium (Ti), tungsten (W), sulfur (S), selenium (Se), and tellurium (Te). The scale bars correspond to 1 μm.

has necessitated the search for materials with high electrochemical activities.41−46 Encouraged by the discovery of remarkably enhanced HET activity and current response of WX2 nanosheets after decorating with TiO2 nanoplatelets, we utilized these nanohybrids in constructing second generation enzymatic biosensors for the identification of H2O2 molecules. For this purpose, the biosensor was constructed as detailed in the Experimental Methods section and schematically shown in Scheme1B. Briefly, the respective TiO2@WX2 was first deposited onto the surface of a glassy carbon (GC) electrode. Then, horseradish peroxidase (HRP) and glutaraldehyde (GTA) was deposited respectively to give GC/TiO2@WX2/ HRP/GTA electrode. HRP is the enzyme of choice due to its promising capability in catalyzing H2O2 and great variety of other substrates, low cost, and high availability.47 HRP contains heme as a prosthetic group, which is also the protein active site with the resting state of the heme iron.48

With TiO2@t-BuLi WS2 selected to be the best nanohybrid material in terms of HET rate, we next moved to investigate the voltammetric responses of [Fe(CN)6]3−/4− in 0.1 M KCl on this nanohybrid with different weight ratios of constructing components. As can be seen in Figure 6A, the hybrid material with TiO2/t-BuLi WS2 weight ratio of 3:1 exhibited the highest current response with negligible difference in HET value in comparison to other ratios. In addition, since ferrocene methanol (FcMeOH) is considered as more stable redox probe in bioapplications, the electrochemical activity of TiO2@ t-BuLi WS2 was investigated in the presence of this redox probe. As can be observed from Figure 6B, TiO2@t-BuLi WS2 nanohybrid clearly outperforms the constructing components as well as bare GC in terms of current response. TiO2 Decorated WX2 Nanosheets for Enzymatic Detection of H2O2. The quest for high-performance electrochemical devices such as electrochemical biosensors E


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Figure 7. Cyclic voltammograms of (A) GC/TiO2@t-BuLi WS2/ HRP/GTA (black), GC/TiO2@n-BuLi WS2/HRP/GTA (red), GC/ TiO2@t-BuLi WSe2/HRP/GTA (blue) and GC/TiO2@n-BuLi WSe2/HRP/GTA (magenta), GC/TiO2@t-BuLi WTe2/HRP/GTA (green), and GC/TiO2@n-BuLi WTe2/HRP/GTA (purple). Loading concentration of each catalyst was 1.25 mg mL−1, and concentration of H2O2 was 1.5 mM. (B) GC/TiO2@t-BuLi WS2/HRP/GTA modified electrode with different loading concentrations of TiO2@tBuLi WS2 (0, 0.625, 1.25, 2.5, 5, and 10 mg mL−1); concentration of H2O2, 2.5 mM. (C) GC (black), GC/TiO2@t-BuLi WS2 (red), GC/ TiO2@t-BuLi WS2/HRP (blue), and GC/TiO2@t-BuLi WS2/HRP/ GTA (magenta): concentration of H2O2, 1.5 mM. Conditions were the following: 2 mM FcMeOH in 1 M phosphate-buffered saline (PBS) (pH 7.2); scan rate of 100 mV s−1. (D) GC/TiO2@t-BuLi WS2/HRP/GTA modified electrode in the presence of different concentrations of H2O2 (0, 0.5, 1, 1.5, 2, 2.5, and 3 mM) with TiO2@ t-BuLi WS2 loading concentration of 5 mg mL−1.

Figure 5. HET activity of 1T phase WX2 nanosheets decorated by TiO2 nanoplatelets. Voltammetric response of (A) WS2-based, (B) WSe2-based, and (C) WTe2-based nanohybrids and their corresponding components in standard ferri/ferrocyanide couple. (D) Summary of peak-to-peak separations of TiO2 nanoplatelets, n-BuLi-exfoliated WX2 nanosheets with corresponding TiO2 decorated nanohybrids, and t-BuLi-exfoliated WX2 nanosheets with corresponding TiO2 decorated nanohybrids. Conditions were the following: 5 mM [Fe(CN)6]3−/4− in 0.1 M KCl; scan rate of 100 mV s−1. Potentials are with respect to the Ag/AgCl reference electrode. 3 mm glassy carbon (GC) electrode was modified with 15 μg of the desired TiO2@WX2 nanocomposite.

H 2O2 + HRP → H 2O + HRP(I)

(1)

HRP(I) + FcMeOH(red) → HRP(II) + FcMeOH(ox) (2)

HRP(II) + FcMeOH(red) → HRP + FcMeOH(ox)

Figure 6. Cyclic voltammograms of (A) TiO2@t-BuLi WS2 hybrid material with TiO2 to t-BuLi WS2 ratios of 3:1 (black), 1:1 (red), and 1:3 (blue) in the presence of 5 mM [Fe(CN)6]3‑/4‑ in 0.1 M KCl, (B) on TiO2@t-BuLi WS2 (black), TiO2 (red), GC (blue), and t-BuLi WS2 (magenta) in 2 mM FcMeOH with 1 M phosphate buffered saline (PBS) (pH 7.2). Experimental conditions were the following: scan rate of 100 mV s−1. The 3 mm glassy carbon (GC) electrode was modified with 15 μg of the desired TiO2@WX2 nanocomposite.

(3) ∼ 100 mV − TiO2 @WX 2

FcMeOH(ox) + 2e ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ FcMeOH(red)

(4)

where FcMeOH (red), FcMeOH (ox), HRP(I), and HRP(II) are the reduced and oxidized forms of FcMeOH (mediator) and the intermediate states of HRP, respectively. In this way, H2O2 was reduced to water in the presence of HRP and the enzyme was reduced to its first intermediate state HRP(I) (eq 1). The HRP(I) was later oxidized to second intermediate HRP(II) in the presence of FcMeOH (red) which in turn was oxidized to FcMeOH (ox) (eq 2). Then, HRP(II) was oxidized to HPR in the presence of FcMeOH(red) (eq 3). Finally, the oxidized FcMeOH was electroreduced by the enhanced electron transfer platform of TiO2@WX2 (eq 4). All six nanohybrid-based biosensing systems exhibited excellent electrocatalytic activity in the presence of H2O2. However, TiO2@t-BuLi WS2 based biosensing system outperformed the other nanohybrid-based biosensors. After the initial evaluation of the efficiency of each TiO2@ WX2 nanohybrid material in the constructed biosensing system and observation of the enhanced performance of TiO2@t-BuLi

Glutaraldehyde (GTA) was used as a cross-linking agent for efficient immobilization of HRP and TiO2@WX2 nanocomposites onto the GC electrode surface.49 Following this, the electrocatalytic activity of second-generation biosensor system based on TiO2@WX2 toward H2O2 detection by using FcMeOH as a mediator was assessed by means of cyclic voltammetry (CV) measurements. For this purpose, HRPbased biosensors were constructed using the nanohybrids and their current responses were evaluated by CV measurements (Figure 7A). Interestingly, a reductive peak at low positive potential was observed in the cathodic scan CV; this peak corresponds to electroreduction of FcMeOH. The chemical reaction mechanism proposed in Scheme 1B can be summarized as follows:47 F


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ACS Applied Nano Materials WS2, the optimum loading concentration of TiO2@t-BuLi WS2 to be deposited on GC was explored (Figure 7B). As can be seen, reduction currents increased as the loading of TiO2@tBuLi WS2 increases from 0 to 1.25 mg mL−1. However, with further increase of loading concentration, the reduction current decreases which can be attributed to the saturation of the electrode surface. The reason for the rather sensitive response can be attributed to the faster electron transfer kinetics of TiO2@tBuLi WS2 nanohybrid and the increased density of catalytically active sites due to the higher contribution of 1T-phase in the tBuLi-assisted WS2 nanosheets. On the other hand, 1T-phase WS2 represent a suitable support for TiO2 nanoplatelets and the resulting nanocomposite shows enhanced surface area and in consequence enhanced H2O2 sensing capability based on their improved HET. In addition, the electrochemical behavior of different modified layers toward the reduction of H2O2 was investigated (Figure 7C). Deposition of TiO2@t-BuLi WS2 on the GC electrode resulted in a slight enhancement in the current response after injection of H2O2. After immobilization of HRP, the reduction current significantly increases which is due to the strong catalytic activity of the HRP. The final immobilization step of glutaraldehyde gave rise to additional increase in the reduction current. Glutaraldehyde not only acts as crosslinking network to preserve the structural integrity of HRP but also holds the TiO2@t-BuLi WS2 and HRP layers on the GC electrode.50 Finally, Figure 7D illustrates the CV curves of GC/TiO2@tBuLi WS2/HRP/GTA biosensor system in different concentrations of H2O2. An almost linear increase in reduction currents can be seen by increasing the concentrations of H2O2 from 0 mM to 3 mM which indicates that HRP enzyme on the TiO2@t-BuLi WS2 nanohybrid retains its electrocatalytic activity for the reduction of H2O2 and can be used for H2O2 sensing. Moving to investigate the analytical parameters of GC/ TiO2@t-BuLi WS2/HRP/GTA biosensor, chronoamperometric measurements were carried out with the addition of different concentrations of H2O2 into stirred FcMeOH (2 mM) in PBS at +0.1 V (Figure 8A). By increasing the concentration of H2O2, reduction currents increase. The dependence of the chronoamperometric response on concentration can be observed on a logarithmic scale (Figure 8B). The calibration graph between absolute reduction current value and concentration of H2O2 demonstrated three independent segments where wide linear region from 0.5 μM to 30 μM (r = 0.98), from 50 μM to 300 μM (r = 0.98), and from 500 μM to 3 mM (r = 0.96) is seen. Furthermore, the limit of detection (LOD) was determined by using LOD = 3s/ m equation. The “m” represents the slope for the calibration curve and “s” indicates the standard deviation of the chronoamperometric current of the lowest concentration of H2O2.51 The system exhibited a low LOD of 8.7 μM which reflects its high sensitivity toward detection of H2O2. The obtained biosensor shows enhanced analytical performance for H2O2 detection with a good LOD comparable with other systems based on AuNPs−MoS2, AuNPs−TiO2 nanocomposites. Moreover, this system shows very wide linear range compared with previous reports (see Table S2). Encouraged by the working capabilities of GC/TiO2@t-BuLi WS2/HRP/GTA biosensor, its feasibility for H2O2 detection in real samples was examined. For this purpose, the influence of

Figure 8. (A) Chronoamperometric measurements of the GC/ TiO2@t-BuLi WS2/HRP/GTA upon successive injections of 0.5 × 10−6, 5 × 10−6, 50 × 10−6, and 500 × 10−6 M H2O2 (in order). Five successive additions of H2O2 were performed for each concentration. (B) Logarithmic relationship between the concentration of H2O2 and the catalytic current. Error bars represent the measures performed using three different biosensors (n = 3). (C) Chronoamperometric response of the GC/TiO2@t-BuLi WS2/HRP/GTA to 100 μM H2O2 in the presence of interferences: 100 μM AA, 100 μM UA, and 100 μM DA. (D) GC/TiO2@t-BuLi WS2/HRP/GTA biosensor for H2O2 detection in human serum. Chronoamperometric measurements upon successive additions of 100 μM H2O2 in FcMeOH (red) and 1:1 FcMeOH/serum (black). Conditions were the following: stirred, 2 mM FcMeOH in phosphate-buffered saline (PBS) (pH 7.2), applied potential of +0.1 V.

interference analytes, i.e., uric acid (UA), ascorbic acid (AA), and dopamine (DA), on the performance of biosensing system was investigated. Figure 8C displays the current responses of the biosensor upon successive injections of 100 μM H2O2, 100 μM AA, 100 μM UA, 500 μM DA, and 100 μM H2O2 into stirred FcMeOH (2 mM) in PBS. The addition of interfering compounds gave rise to negligible current changes, while a well-defined response is observed with the additions of H2O2. This excellent selectivity can be assigned to the fact that the biosensing system likely reacts preferentially with H2O2 rather than other compounds. Furthermore, practical applicability of the GC/TiO2@t-BuLi WS2/HRP/GTA biosensor for detection of H2O2 in human serum was investigated. As can be seen from Figure 8D, successive injections of 100 μM H2O2 to the mixture of FcMeOH and human serum with volume ratio of 1:1 exhibited a similar response when the same concentration of H2O2 was added to the FcMeOH solution.

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CONCLUSION We reported remarkable enhancement in the current response and heterogeneous electron transfer capability of lithiumexfoliated tungsten dichalcogenide (WX2) nanosheets through decorating with vertically aligned TiO2 nanoplatelets. These significant improvements can be attributed to the outstanding features of these novel nanohybrids, such as homogeneous decoration of TiO2 nanoplatelets on the surface of WX2 nanosheets, strong electrostatic interaction between WX2 nanosheets and decorated TiO2 nanoplatelets, vertical orientation of TiO2 nanoplatelets with increased number of electrochemically active sites, and metallic nature of 1T-phase G


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samples were suspended in deionized water (1 mg/mL) and ultrasonicated for 10 min. The suspension was deposited on a small piece of silicon wafer and dried. Voltammetry and chronoamperometry measurements were conducted on a μAutolab type III electrochemical analyzer (Eco Chemie, The Netherlands). Data were collected by NOVA software. A standard three-electrode setup was employed for performing electrochemical measurements at room temperature. Ag/AgCl electrode, platinum electrode, and glassy carbon electrode (GC) of 3 mm in diameter served as the reference electrode, the auxiliary electrode, and the working electrode. All the electrodes were purchased from CH Instruments, Inc., USA. Procedures. Synthesis of Bulk Tungsten Ditelluride and Diselenide Crystals. Stoichiometric amounts of tungsten metal and tellurium (and selenium for WSe2) corresponding to 10 g of the final product were put in quartz ampule (100 mm long, 15 mm outside diameter, wall thickness 2 mm) and pumped to vacuum level of 1 × 10−3 Pa. Afterward, the ampule was melt-sealed using oxygen− hydrogen welding torch, and then it was heated (5 °C/min) to 600 °C for 48 h. Subsequently, ampule content was mixed for 10 min followed by heating (5 °C/min) to 800 °C for 48 h and finally to 850 °C for 12 h and then cooled back to room temperature at the same rate. Exfoliation of Tungsten Dichalcogenides (WX2). Exfoliation of bulk WX2 (X = S, Se, and Te) crystals was carried out based on the previously reported technique.52 To prepare the suspensions, a particular amount of WX2 bulk powder (3 g) was added to 20 mL of tert-butyllithium (1.7 M in pentane) and 20 mL of n-butyllithium (1.6 M in hexane). Then, the mixtures were stirred at room temperature under argon atmosphere for 72 h. The lithium intercalated WX2 was isolated using suction filtration under an argon atmosphere and then rinsed repeatedly with hexane (dried over sodium). The delaminated nanosheets were subsequently dispersed in 100 mL of water and bathsonicated for 10 min. Purification by centrifugation (18 000g) and redispersion was performed repeatedly until the conductivity fell below 20 μS. Finally, the exfoliated WX2 was dried under vacuum. Synthesis of TiO2 Nanoplatelets. Two-dimensional nanoplatelets of TiO2 were prepared according to a well-established procedure reported previously.36 In brief, a 1.05 g portion of titanium isopropoxide (TTIP) was added into 0.74 g of concentrated HCl solution under vigorous stirring for 15 min. PEO20−PPO70−PEO20 (Pluronic P123, 0.2 g) was dissolved in 3.0 g of ethanol. Then, the solutions were mixed and stirred for another 30 min. 2.5 mL of obtained mixture with 20 mL of ethylene glycol was transferred into a Teflon-lined stainless-steel autoclave and sealed and heated at 150 °C for 20 h. The product of the reaction was washed thoroughly with ethanol, and the resulting white powder was collected after drying at 80 °C for 24 h. Preparation of PEI-Modified TiO2 Nanoplatelets. The polycation modification process of TiO2 nanoplatelets is as follows. The aqueous TiO2 nanoplatelets suspension (5 mg/mL) was slowly dropped into the polyethylenimine aqueous solution (PEI, Mw = 7.5 × 105, 50 mg/ mL), and the resultant mixture was left to stir overnight. The excess polymer was removed by repeating centrifugation at 14 000 rpm for 30 min and washing cycles twice. Finally, the PEI-modified TiO2 nanoplatelets were redispersed in water. Preparation of TiO2@WX2 Nanocomposites. Flocculation of positively charged nanosheets (PEI-modified TiO2 nanoplatelets) with negatively charged nanosheets (WX2) was performed using a previously reported method.38 A designed volume of PEI-modified TiO2 nanoplatelets suspension was slowly added into the exfoliated WX2 suspension under continuous stirring. The resulting flocculated material was collected via centrifugation at 2000 rpm for 10 min and dried in air. Biosensor Preparation. The 3 mm glassy carbon (GC) electrodes were polished with 5 and 0.05 μm Al2O3 particle slurry for renewal of the surface. Then, the electrode was rinsed thoroughly with deionized water and dried under a N2 gas stream. Prior to the electrochemical measurements, suspensions of exfoliated WX2, TiO2 nanoplatelets, and TiO2@WX2 nanocomposites (5 mg mL−1) were ultrasonicated

few-layered WX2 materials. Interestingly, it was found that the nanohybrids constructed with tert-butyllithium exfoliated WX2 materials exhibited faster HET rates and higher current responses in comparison to the n-butyllithium exfoliated counterparts which is attributable to the predominant metallic 1T-phase of tert-butyllithium exfoliated WX2. Furthermore, the HET activity and current intensity of nanohybrids showed an obvious dependence on the type of chalcogen (S > Se > Te) as well as the relative mass ratio of constructing components (TiO2/WX2, 3:1 > 1:1 > 1:3). As a result, TiO2@t-BuLi WS2 nanohybrid, with a mass ratio of 3:1, demonstrated the highest redox current intensity and the fastest HET rate among all the nanohybrids. With these extremely improved electrochemical features in hand, all the nanohybrids were then used in construction of enzymatic-based H2O2 biosensor and demonstrated good performance in detection of H2O2, with the highest electrocatalytic activity toward the reduction of H2O2 observed for TiO2@t-BuLi WS2. By optimization of the experimental conditions, the biosensor displayed a wide linear range, low detection limit, and high selectivity. It is believed that the obtained nanohybrids with excellent electrochemical activities can be utilized in other electrochemical application. In addition, this strategy will open up a new paradigm in improving the electrochemical performance of not only twodimensional (2D) transition metal dichalcogenides but also other 2D materials which can broaden their functionality in electrochemical applications.

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EXPERIMENTAL METHODS

Materials. Tungsten disulfide (99.8%), selenium (99.999%), and tungsten powder (99.9%) were purchased from Alfa Aesar, Germany. Tellurium (99.999%) was purchased from Mateck, Germany. tertButyllithium (1.7 M in pentane) and n-butyllithium (1.6 M in hexane) were obtained from Sigma-Aldrich, Czech Republic. Argon (99.999% purity) was obtained from SIAD, Czech Republic. Potassium hexacyanoferrate(II) trihydrate (≥98.5%), potassium hexacyanoferrate(III) (≈99%), monobasic potassium phosphate (≥99.0%), dibasic sodium phosphate (≥99.0%), phosphate buffered saline tablet, human serum, horseradish peroxidase, glutaraldehyde solution (≈70% in water), potassium chloride (max 0.0001% Al), titanium isopropoxide (≥97.0%), concentrated hydrochloric acid (37%), polyethylene oxide−polypropylene oxide−polyethylene oxide (PEO20−PPO70−PEO20, Pluronic P123), polyethylenimine solution (∼50% in H2O) were purchased from Sigma-Aldrich, Singapore. Hexane was purchased from Lach-Ner, Czech Republic. Hydrogen peroxide (35% w/w aqueous solution), ethylene glycol (99%), and hydroxymethylferrocene (ferrocene methanol) (97%) were purchased from Alfa Aesar, Singapore. Deionized water with a resistivity of 18 MΩ cm was used through all the experimental procedures. Apparatus. Sample structure was characterized using fieldemission scanning electron microscope (JEOL 7600F, Japan) at an acceleration voltage of 5 kV. Transmission electron microscopy (TEM and high-resolution) images were taken with FETEM JEOL 2200 FS microscope (JEOL, Japan) operating at 200 eV. Samples for TEM investigation were prepared by drop casting the sample suspension (1 mg/mL) on a 200 mesh TEM grid. Elemental mapping was performed with an SDD 149 detector X-MaxN 80 T S (Oxford Instruments, England). X-ray diffraction analysis was carried out using Bruker D8 Discoverer (Bruker, Germany) powder diffractometer with Cu Kα radiation (λ = 0.154 18 nm, U = 40 kV, I = 40 mA); step scan 0.019°, 10−80° 2θ range. Raman spectroscopy was performed with an inVia Raman microscope (Renishaw, England) in backscattering geometry with CCD detector and with 532 nm excitation laser. The Raman emission was collected by 50× objective lens. Instrument calibration was achieved with a silicon reference which gives a peak position at 520 cm−1 and a resolution of less than 1 cm−1. The H


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ACS Applied Nano Materials for a period of 1 h to ensure homogeneous dispersions. 3 μL aliquot from each dispersion was then drop casted on the electrode surface and allowed to dry. Subsequently, 5 μL of 1 mg/50 μL horseradish peroxidase (HRP) solution prepared in PBS, pH 7.2, was deposited on the glassy carbon electrode modified with TiO2@WX2 and left to dry at room temperature. Finally, 5 μL of aqueous glutaraldehyde solution (0.25% v/v) was drop casted to the electrode surface to cross-link the HRP enzyme and allowed to dry in an oven at 35 °C. When not in use, the electrode was kept at 4 °C. Electrochemical Measurements. Cyclic voltammetry (CV) for HET measurements was carried out at a scan rate of 100 mV s−1 using 5 mM ferri/ferrocyanide redox probe in 0.1 M KCl supporting electrolyte. HET rate constant (k0obs) was calculated using the Nicholson method, relating the ΔEp to a dimensionless parameter, ψ, and consequently to kobs 0 . The roughness factor was not considered in this work. The [Fe(CN)6]3−/4− diffusion coefficient of 7.26 × 10−6 cm2 s−1 was used for calculation. For hydrogen peroxide detection, all the measurements were carried out in 2 mM ferrocene methanol (FcMeOH) in the presence of 1 M phosphate buffered saline (PBS), pH 7.2, solution. Cyclic voltammograms were collected at a scan rate of 100 mV s−1, and chronoamperometry measurements were recorded at a constant potential of +0.1 V under stirring conditions. For further investigation, the baseline of chronoamperometry results was corrected after acquisition.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsanm.8b01796. SEM images of WX2 materials, XRD patterns of WX2 materials; tabulated HET rate constants of WX 2 nanosheets, TiO2 nanoplatelets, and TiO2@WX2 nanohybrids; table for comparison of performance parameters of nanostructured electrochemical H2O2 biosensors (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Carmen C. Mayorga-Martinez: 0000-0003-3687-0035 Rasoul Malekfar: 0000-0001-5529-5983 Zdenek Sofer: 0000-0002-1391-4448 Martin Pumera: 0000-0001-5846-2951 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the project Advanced Functional Nanorobots (Reg. No. CZ.02.1.01/0.0/0.0/15_003/0000444 financed by the EFRR). E.R. and R.M. were supported by Iran Science Elites Federation and Ministry of Science, Research and Technology of the I. R. Iran. Z.S. and J.L. were supported by Czech Science Foundation (GACR Grant No. 17-11456S). J.L. was supported by specific university research (MSMT Grant No. 20-SVV/2017). This work was created with the financial support of the Neuron Foundation for Science Support.

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REFERENCES

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